How are elementary particles detected?
The only way to find a particle is to observe it in its interaction with others. Incoming particles are inserted into an accelerator and sent to collide. Out of that collision, new particles emerge and are set to collide again but this time with a detector which is essentially a calorimeter designed to stop the particle entirely. Its energy is converted into heat and from the amount of heat one obtains the mass of the particle.
Electric charge is found by tracking the particle's path inside the detector. Positive charges rotate in the opposite direction as negative charges when placed in a magnetic field.